Huge water pulse to bring Colorado river back from dead

A historic US-Mexico deal will see 130 billion litres of water infuse the river's parched delta, leaving an explosion of life in its wake

FROM above it looks like a meandering yellow scar. On either side lie lush green polygons – the irrigated fields of the Mexicali valley just south of the US border, where tomatoes, cucumbers and onions grow in what should be a desert. The dusty smear running in between used to be the Colorado river. It's been decades since it has reached the sea.

On 23 March, it will begin its journey back from the dead. In an ecological experiment unprecedented in US history, seven states and two countries have signed an agreement to unleash a huge pulse of water designed to bring the river's dwindling delta back to life.

"We're trying to engineer a spring flood," says Karl Flessa, a geoscientist at the University of Arizona who is leading the team that will study how the delta responds. "This is a river system that historically had a huge spring flood every year. We're trying to recreate that."

Before development of the American south-west led to the widespread damming of the Colorado from the early 1900s onwards, the floods that fed the river's delta kept it teeming with life. When explorer Aldo Leopold canoed the delta in 1922, he marvelled at "a verdant wall of mesquite and willow" that separated the river from the desert. Jaguars prowled outside the few human camps that existed.

To try to bring back this lost ecosystem, Flessa and his team are planning to release a one-time, 130-billion-litre pulse of water.

The water that will make up the pulse is currently being slowly released from behind the Hoover, Davis and Parker dams. It will collect behind the most southerly dam on the river, the Morelos, which sits on the Mexico-US border and normally diverts the last of the Colorado toward agricultural land.

When the gates of the Morelos open on 23 March, the river will be reunited with its delta. The eight-week-long pulse will release enough water into the dry riverbed to fill an area the size of a Manhattan city block with a column six kilometres high.

After that, the agreement stipulates that a small continuous flow, totalling an additional 64 billion litres, will infuse the delta over the next three years. It's a trickle compared with what used to reach the delta, but researchers still expect the water to bring around 950 hectares of the delta to life in the weeks after the pulse.

The experiment isn't just remarkable for its scale. It is also the first time water has crossed the US-Mexico border for environmental purposes – the result of years of negotiations between Mexican and US water authorities, as well as a host of NGOs. "As far as I know there has never been an agreement to deliver water for biological purposes," says Michael Cohen of the Pacific Institute in Oakland, California. "It's been in the works for arguably 20 years."

Water rights

The agreement, called Minute 319, requires the US to provide $21 million to help improve water-saving measures in Mexico, and for Mexico to forego some of its water rights this year to free up water for the pulse. It also provides a legal framework designed to forestall conflict over how much water the US lets flow into Mexico, especially during droughts like the one currently gripping the region. This could prove vital in the coming years as climate models suggest the region will grow increasingly arid.

When the waters hit, scientists expect the Colorado river delta to undergo an explosive transformation. It has happened before. El Niño dumped so much rain on the region for a few scattered years in the 1980s and 90s that officials were forced to let some water flow to the ocean.

Ed Glenn of the University of Arizona studied the impact on the delta, and saw native trees growing at speeds that seem otherworldly. "There's an immediate green response in the satellite imagery," he says. "You're germinating new generations of all kinds of plants, including the native trees that are so valuable to birds."

The hope is that this time, with a consistent base flow following behind the initial pulse, life will be there to stay. One of the main goals is to encourage native cottonwood and willow trees back into the delta. Right now, sticky mud flats are dominated by salt cedar, an invasive species.

Cottonwoods are uniquely adapted to make the most of flood waters. "The cottonwoods are able to extend their roots as much as a centimetre a day during the first growing season," says Glenn, allowing them to continue to tap groundwater stores even after the pulse is over.

Flessa and his team are planning to make the most of the opportunity to use the delta as a massive living experiment. They have buried temperature and moisture sensors across the flood plain to measure exactly when the pulse arrives. Another set of sensors, buried 10 metres deep in the silty soil, will measure how quickly the water seeps in. The data gathered will be combined with laser scans of the flood plain and satellite imagery of vegetation to determine exactly how the water restores the area.

If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.

Water from behind the Hoover dam will help life in the river delta (Image: Derek E. Rothchild/Image Bank/Getty)